1. Field of the Invention
The present invention relates to a clock combining circuit used, for example, in an optical disk apparatus that accomplishes recording and playback of user data on an optical disk.
2. Description of Related Art
Edge recording methods that record data on a recording medium by corresponding 1s in the data stream to the positions of the positive edges and negative edges of a recording signal, and that accomplish data decoding by detecting the positive edges and negative edges of the playback signal waveform are utilized, for example, by ISO 5.25 inch quadruple density magnetooptical disk drives in order to achieve high density recording. When data is recorded through this kind of edge recording method, the data is recorded by converting the data into 1.7-modulation run length limited code and causing the positive edges and negative edges of the recording signal to correspond to edge positions of a condition change pattern (i.e., a mark) formed on the recording medium by pits or the like. During playback, a clock is created that is synchronized with the positive edge or with the negative edge of the playback signal waveform obtained from the recording medium. The binarized waveform of the playback signal is sampled using this clock and the binarized data indicating a "1" at the positions of positive edges and negative edges in the waveform is played back. Furthermore, data is played back by conducting data decoding after obtaining the byte synchronization from mark detection such as SYNC or the like.
However, when data recording is accomplished through edge recording methods, there are cases when, for example, the front edges and back edges of the pits do not have the desired spacing. This occurs, for example, due to the effects of fluctuations in the recording conditions and thermal capacity of the recording medium when the pits are provided by irradiating the optical disk with laser light. In other words, this causes the pit length to fluctuate. For example, when a pit is provided after a long blank, the position of the front edge is shifted to a delayed position. Additionally, when a blank is provided after a long pit, the back edge of the pit is shifted to a delayed position. When these kinds of shifts occur, a discrepancy is created in the relative positional relationship of the positive edge and negative edge of the playback signal, that is, a phase error is created. When a phase error is created, it is difficult to playback accurately the data near the negative edge when data playback is accomplished using a clock that is synchronized with the positive edge. It also is difficult to playback accurately the data near the positive edge when data playback is accomplished using a clock that is synchronized with the negative edge.
Accordingly, it is necessary to correct for shifts in the front edges and back edges during recording.